I do not know if this will be the same for player controlled orc encampments and cities but I have been on a quest which requires me to attack two orc cities one or two walls thick and as elves I prefer to use more rangers in my army. I have tried twice now and lost many units and gold in healing (which is not hte largest deal) but there seems to be a bug where my arrows which are flamming or standard cannot hit the orcs on their battlements. One further thing from this is that my hero unit and many of my other units don't seem to be able to go onto large towers, only the Twin bladed swordsmen seemed to be able to kill the enemy catapults and archers etc on the tall towers.

If your units are too close to the walls, they won't be able to arc their arrows to hit the units on top. Try changing the position of your units and/or spending Skill Points to increase their range.

Don't just rely on Rangers, though. Try sending in Treants to attach to the walls and have your Sentries and Blademasters scale them to take out the defenders, while your Rangers pepper them with arrows from a distance.

All infantry units should be able to climb towers, sometimes their path might be blocked by other units though.

I have used arrow units from all different positions. I have not attacked a settlement recently but during my attack there was 0 damage done. I used fire arrows in a follow-up attack on Nok Rassol and there again was no damage vs the orc archers. I have attacked both Elven and Human Settlements and Castles and both types of sieges I was able to kill the enemy units on the walls including archers and siege weapons. However none of the orc units have ever taken a single arrow from my rangers, i have not tried with Female wardens but I do not build them.

I will try to get more information but as of now the seiges vs Orcs is much harders as I can only use melee units. I first have to destroy gates and walls (since orc only have wooden walls) and simply smash my way through.